1. Field of the Invention
This invention relates to an artificial stem for femur of coxa in a human body.
2. Description of the Prior Art
A conventional artificial stem for femus or coxa is disclosed, for example, in Japanese Laid-open Patent Publication No. 41963 of 1985 (Toku-kai-sho 60-41963).
This artificial stem comprises, as shown in FIG. 6(B) (side elevation), a stem element 13 curving slightly backward (leftward in the drawing) in the longitudinal direction of an artificial stem 1, a small head 14 and a neck 15. The stem element 13 comprises an upper proximal portion 11 and a lower distal portion 12. Preferably, the distal portion 12 should be approximately two to three times longer than the proximal portion 11. Numeral 16 designates a shoulder 16. The proximal portion 11 is slightly twisted at an angle of approximately 5 to 15 degrees, as shown in FIG. 6(C). Preferably, the twist should extend throughout the entirety of the proximal portion 11, at an angle of approximately 9 degrees. Also, the proximal portion 11 should preferably be covered with a porous surface 16 to provide good stimulus to the growth of bone.
The proximal portion 11 of the stem elements 13 can be divided, from top to bottom, a proximal, a middle and a distal section. The distal section has a lateral flare that becomes wedge-interlocked with the femur when the stem element 13 is inserted.
The neck 15 diagonally extends upward from the upper proximal section of the proximal portion 11, with the axis thereof forming an angle of approximately 45 degrees with the axis of the stem element 13. The small head 14 is fastened to the neck 15 by taper driving.
The artificial stem for femur of coxa of the above-described type has the following problems:
(1) Designed to fit the femur and medullary cavity of big-body people the artificial stem does not always fit those of small-body people. With the small-body people, therefore, the above artifical stem does not seem to occupy as much space in the bone cavity as is described in the specification.
(2) For the reason described above in (1), it is doubtful if the artificial stem fits in the center of the bond cavity when the stem is set in portion. If it does not fit properly, the thickness of the cement may vary greatly in some places. When a load is placed on the artificial stem, localized stress concentration may result to cause loosening.
The curvature of the stem not fitting the bone cavity of small-bodied people necessitates an additional process of enlarging the insertion hole with file or the like.
(3) The cross-sectional shape of the stem is not always rigid enough to withstand the torsion under a load (like a twist).
(4) The lateral flare may have a wedging effect, it also creates a localized stress concentration that, in turn, can lead to loosening or damages to the bone.